Speciation Specifics

The golden whistler (Pachycephala pectoralis), richer in geographic variation than any other bird, is the world's greatest speciator. Sixteen of its 66 well-marked subspecies are found in Northern Melanesia (the 200 or so islands of the Bismarck and Solomon archipelagoes, which lie east of New Guinea, northeast of Australia and just south of the equator). Across these islands, another 30 bird species have populations well differentiated enough that they too are considered great speciators—that is, they have seven or more geographically distinct populations, or they have at least four distinct populations spread over no more than 12 islands. What factors make birds speciate, and why should these archipelagoes be particularly conducive to this process?

In The Birds of Northern Melanesia, Ernst Mayr and Jared Diamond examine the levels of geographic differentiation within each of the bird species in the two archipelagoes. The authors' comparisons between species provide multiple snapshots of the speciation process and uncover factors favoring differentiation.

This is a handbook, with many tables and lengthy descriptive passages. It includes 52 maps with detailed information regarding species distribution and nine color plates illustrating some of the more distinctive taxa. Mayr and Diamond identify general patterns—for example, in the species-area relationship across islands, in ultimate sources for the avifauna and in colonization routes. In addition, each species and island has its own evolutionary history, and analyses of special cases provide support for many of the authors' theses, as well as puzzles to be solved. The book is something to be consulted rather than read through, but many interesting ideas are hidden among the facts.

The unit of analysis is the zoogeographic species. Some zoogeographic species consist of two or more related populations that are allopatric in distribution (geographically separate) and are inferred to be reproductively isolated from each other. Such allospecies are ordinarily counted as separate species, but focusing on zoogeographic species instead makes sense, given the authors' emphasis on the continuum of allopatric forms and the insights such gradations provide into the process of speciation.

In Northern Melanesia, 191 of the zoogeographic species are native, breeding residents. (Using the usual construction of the term species would increase the tally to 251.) Ninety-one zoogeographic species are undifferentiated throughout their Northern Melanesian range, and the rest are divided into subspecies, megasubspecies and allospecies, largely on the basis of plumage variation. (A megasubspecies is a taxonomic category that indicates a well-defined subspecies or group of subspecies whose distinctness approaches the allospecies level.)

Mayr and Diamond consider the very last stage of the speciation process to be geographical overlap between a pair of species derived from the same stock. They show that this is a rare event in Northern Melanesia: Only 19 such pairs of species are found there. For these sympatric pairs, each species appears to be the result of a separate invasion from a single source species outside Northern Melanesia. The one exception is a sympatric pair of species of white-eyes (Zosterops murphyi and Z. rendovae), which probably both formed within the Solomons. In more isolated archipelagoes, notably the Galápagos and Hawaii, birds radiating from a single ancestral stock within the archipelago have occupied diverse ecological niches. But in Northern Melanesia, the proximity of New Guinea—with its large number of ecologically different species readily available to disperse into the Bismarck and Solomon archipelagoes—apparently has made ecological diversification within the archipelagoes very difficult.

Thus not only is sympatric speciation entirely absent, even the achievement of sympatry among closely related species is extremely rare. Consequently Mayr and Diamond put little emphasis on how adaptation to island environments may have directly promoted divergence, but they do mention a few examples in which divergence may have been fostered by ecological conditions on an island. First, they note that Long Island, subjected to volcanic defaunation in the 17th century and then recolonized, is now populated by four subspecies (the cuckoo Eudynamys scolopacea, the swallow Hirundo tahitica, and two types of honey-eaters—Myzomela pammelaena and M. sclateri) that are weakly differentiated from their parent populations. Ecological interactions between the colonizing species (character displacement) are one possible explanation for this differentiation. Second, isolated peripheral populations are often the most distinctive, and low species diversity on such islands may give greater opportunities for different selection pressures in association with ecological variation. In addition, genetic explanations are offered for some of the patterns: Founder events may promote divergence, and large population size may speed up evolution because more adaptive mutations appear.

These ideas are mentioned only in passing. The book's main emphasis is on the twin roles of isolation and time in promoting population differentiation to the level of full species. The data provide strong support for the primacy of these two factors. First, consider analysis at the level of the island. If one excludes recent volcanoes, along with islands believed to have been joined to one another by land bridges during the Pleistocene, then island area and distance from the nearest colonization source explain 86 percent of the variance in an index that weights an island's taxa by the degree of endemism (giving, for example, less weight to differentiation at the level of subspecies than to differentiation at the level of allospecies). The reason that larger areas promote divergence is thought to lie in population persistence: A long time is needed for differentiation to the level of full species; larger areas have higher population sizes and hence are less prone to extinction. The authors' clever analysis of data from the islands created when Greater Bukida (the largest Pleistocene island) was fragmented by rising sea levels shows that no nondispersing population has survived on an island smaller than 5 square kilometers. Even large islands of about 1,000 square kilometers have lost species.

Analysis at the species level gives similar results. Mayr and Diamond classify species into different dispersal classes: Those that have historically been observed dispersing are most vagile, those that have colonized recent volcanoes are of intermediate vagility, and those for which there is no evidence of dispersal during the lifetime of the species are of low vagility. They also classify species according to abundance (based on crude estimates of population density and suitable habitat). When they controlled for the number of islands on which a given species exists, they found both abundance and vagility to be correlated with a measure of geographic variation within the species. Species of high vagility and low density have the lowest amount of geographic variation. However, dispersal has dual opposing effects: Some dispersal is needed to colonize islands. Those 31 "great speciators" with many well-marked populations, such as the golden whistler, fall disproportionately into the intermediate-vagility, high-abundance class.

Finally, the Solomons have a greater number of endemic zoogeographic species than the Bismarcks, despite having fewer species in total (about 122, versus 142 on the Bismarcks, excluding geographically peripheral species) and less geographic variation on average within species. All five of the monotypic genera (genera containing a single species) endemic to Northern Melanesia are confined to the Solomons. The authors' explanations again mainly revolve around time and dispersal. They argue, for example, that endemic species may survive longer on the Solomons than on the Bismarcks because the Solomons are farther from the main colonization source (New Guinea), which means fewer invading species to compete with and less gene flow during the early stages of differentiation.

Differentiation is promoted by low vagility and by distance from potential sources of colonists. Why might this be? The usual explanation—one offered frequently in this book—is that gene flow prevents population differentiation. It is an argument seemingly at variance with theory and empirical observations suggesting that natural selection can overcome what must in absolute terms be a very small number of immigrants. Two other explanations are briefly mentioned, in which dispersal is directly linked to population persistence: First, isolated islands have few species, reflecting low levels of successful immigration. This may raise population sizes of the residents and hence their resistance to extinction. Second, and noted in connection with the large numbers of ancient endemic species in the Solomons, is the idea that successful colonization by a new species actually causes extinction of residents. For example, immigrants may be superior competitors or may introduce disease. If dispersal is more frequent, successful immigration must be higher. This means that population turnover will occur at a higher rate, and fewer populations will persist long enough to differentiate to the level of species. This idea has much merit and deserves more study.

Both abundance and propensity to disperse evolve as a consequence of adaptation to the prevailing conditions. Vagility in particular seems to be quickly lost in some habitats (for example, montane forest). The authors list many examples of different subspecies—or even different populations of the same species—showing different tendencies to disperse. For example, six species in the Solomons are classified as highly vagile, whereas the same species in the Bismarcks have never been recorded off their native island. Because high vagility promotes dispersal onto new islands and low vagility promotes differentiation between islands, those species that are initially highly dispersive but rapidly evolve a sedentary habit should be the ones most prone to future speciation events. Much will be gained from further research into the adaptive causes and consequences of dispersal.

In the rush to embrace so-called ecological models of speciation in which adaptation to different environments promotes population divergence, the overriding importance of time and barriers to dispersal in speciation either has been ignored or has been thought to be so straightforward that few details are left to be worked out. This book is a brilliant reminder that there is much to be learned. It marks an important advance in the study of speciation, setting the stage for molecular studies and detailed fieldwork on a variety of species groups.